Drinking water heavy metals sensor and methods for use thereof

ABSTRACT

A sensor for detecting heavy metals in water is provided. The sensor includes a first electrode and a second electrode, the first electrode and the second electrode having complementary interdigitated surfaces that are separated from each other by a first gap having a distance of greater than or equal to about 500 nm to less than or equal to about 10 μm. The sensor also includes a power source connectable to the first electrode and the second electrode. The sensor is configured to continuously monitor water for the presence of heavy metals. Methods of making and using the sensor are also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/509,537, filed on May 22, 2017. The entire disclosure of the aboveapplication is incorporated herein by reference.

FIELD

The present disclosure relates to sensors that monitor home water forheavy metals, such as lead.

BACKGROUND

This section provides background information related to the presentdisclosure which is not necessarily prior art.

Heavy metals, such as lead, in drinking water are dangerous to humans,and regulations for the maximum allowable concentrations of these metalsin drinking water have been established to protect consumers. Leadcauses neurological damage even at low levels of lead exposure,especially in infants and children. The Environmental Protection Agency(EPA) states that zero lead is allowed in maximum contaminant level(MCL), and 15 ppb of lead is listed as the action level. In addition tolead, copper is another dangerous heavy metal that causes liver andkidney damage after long-term exposure. The MCL for copper is 1.3 mg/Land the secondary maximum contaminant levels (SMCL) is 1.0 mg/L. SMCLssuggest ions that cause bad taste, color, and odor should be minimizedin drinking water. Zinc and iron are other two common elements indrinking water that are regulated by SMCLs of 5 mg/L and 0.3 mg/L,respectively.

Lead leakage into tap water is a major concern in the U.S. Houses in theU.S. built before 1986 commonly contain lead in the service lines orvalves. When water flows through these lead components, lead can leachinto the water through a variety of complex electrochemical,geochemical, and hydraulic mechanisms. The leaching often occurs withoutthe awareness of the users because lead can be colorless and odorless.Thus users are at risk from lead exposure through contaminated water ifthe metal contaminant is not detected.

Early detection of lead is important to prevent long-term exposure, butis difficult to achieve using current technology. Because water iscontaminated inside the structure of a house, end-point detection byhome-monitoring is crucial for lead leakage detection. The onlyqualified method suggested by EPA is inductively coupled plasma massspectrometry (ICPMS) at qualified national testing labs. Because leadleakage typically happens unexpectedly, the suggested method requiresthe self-awareness of the users to regularly send water out forexamination. Although minimized sensors for home-monitoring have beenproposed using electrochemical potentialmetric or colorimetric methods,most potentialmetric sensors have short lifetimes due to the limitationof minimized reference electrodes, and colorimetric sensors aretypically single use. Accordingly, there remains a strong need todevelop sensors that detect metals in water and that can operate for along time without input from a user.

SUMMARY

This section provides a general summary of the disclosure, and is not acomprehensive disclosure of its full scope or all of its features.

The current technology provides a sensor for detecting heavy metals inwater. The sensor includes a first electrode and a second electrode, thefirst electrode and the second electrode having complementaryinterdigitated surfaces that are separated from each other by a firstgap having a distance of greater than or equal to about 500 nm to lessthan or equal to about 10 μm; and a power source connectable to thefirst electrode and the second electrode. The sensor is configured tocontinuously monitor water for the presence of heavy metals.

In one various, the first electrode is a positive electrode and thesecond electrode is a negative electrode.

In one variation, the first electrode has a surface area of greater thanor equal to about 0.4 mm² to less than or equal to about 0.5 mm² and thesecond electrode has a surface area of greater than or equal to about0.3 mm² to less than or equal to about 0.4 mm².

In one variation, the sensor further includes a third electrode and afourth electrode, the third electrode and the fourth electrode havingcomplementary interdigitated surfaces that are separated from each otherby a distance of greater than or equal to about 500 nm to less than orequal to about 10 μm. The second electrode and the third electrode areseparated from each other by a second gap having a distance of greaterthan or equal to about 1 μm to less than or equal to about 1 mm.

In one variation, the first electrode is a positive electrode and thefourth electrode is a negative electrode.

In one variation, the sensor further includes a first lead electricallyconnected to the first electrode; a second lead electrically connectedto the second electrode; a third lead electrically connected to thethird electrode; and a fourth lead electrically connected to the fourthelectrode, wherein the sensor is configured such that the first, second,third and fourth leads can be individually coupled to and decoupled fromthe power source.

In one variation, the power source is a battery, a plurality ofbatteries, a photovoltaic device, or an electrical service of abuilding.

In one variation, the sensor continuously and selectively detects leadin water.

In one variation, the sensor is free of a reference electrode and aligand.

In one variation, the sensor also includes a substrate portion coupledto the first and second electrodes by way of an adhesive layer disposedbetween the substrate portion and the first and second electrodes.

In one variation, a water pipe having an internal bore section throughwhich water flows, wherein the sensor is disposed within the internalbore section, is provided.

The current technology also provides a method of continuously monitoringwater for the presence of lead. The method includes contacting a sensorwith a water sample. The sensor includes a first electrode and a secondelectrode, the first electrode and the second electrode havingcomplementary surfaces that are separated from each other by a distanceof greater than or equal to about 500 nm to less than or equal to about10 μm; a third electrode and a fourth electrode, the third electrode andthe fourth electrode having complementary surfaces that are separatedfrom each other by a distance of greater than or equal to about 500 nmto less than or equal to about 10 μm, wherein the second electrode andthe third electrode are separated from each other by a distance ofgreater than or equal to about 1 μm to less than or equal to about 1 mm.The method also includes applying a first electrical potential betweenthe first and fourth electrodes; applying a second electrical potentialbetween the first and second electrodes; measuring a first voltagebetween the first and second electrodes; and determining that lead ispresent in the water sample when comparing the first voltage to abaseline voltage in water that does not contain detectable levels oflead.

In one variation, the water sample is contained in a water pipe.

In one variation, the method further includes generating an alert whenthe first voltage is different from the baseline voltage.

In one variation, the method further includes, after the measuring avoltage between the first and second electrodes, applying a thirdelectrical potential between the third and fourth electrodes; measuringa second voltage between the third and fourth electrodes; anddetermining that heavy metals other than lead are present in the waterwhen comparing the second voltage to second baseline voltage in waterthat does not contain detectable levels of heavy metals other than lead.

In one variation, the sensor is free of a reference electrode or aligand.

The current technology also includes a method of fabricating anelectrode that selectively detects lead in water. The method includesdisposing an adhesive layer on a substrate; disposing a photoresist ontothe adhesive layer; and disposing a photoresist mask on the photoresist,wherein the photoresist mask includes a pattern. The pattern defines afirst electrode and a second electrode, the first electrode and thesecond electrode having complementary surfaces that are separated fromeach other by a distance of greater than or equal to about 500 nm toless than or equal to about 10 μm; and a third electrode and a fourthelectrode, the third electrode and the fourth electrode havingcomplementary surfaces that are separated from each other by a distanceof greater than or equal to about 500 nm to less than or equal to about10 μm, wherein the second electrode and the third electrode areseparated from each other by a distance of greater than or equal toabout 1 μm to less than or equal to about 1 mm. The method also includestransferring the pattern of the photoresist mask into the adhesive layerto generate a patterned adhesive layer; and disposing a layer of aconductive material onto the patterned adhesive layer.

In one variation, the adhesive layer includes titanium, platinum, or acombination thereof.

In one variation, the conductive material includes platinum, gold,silver, copper, or a combination thereof.

In one variation, the substrate includes silicon dioxide.

Further areas of applicability will become apparent from the descriptionprovided herein. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure.

FIG. 1 is an illustration of a two-electrode sensor according to variousaspects of the current technology.

FIG. 2 is an illustration of a four-electrode sensor according tovarious aspects of the current technology.

FIG. 3A is a photograph of an exemplary two-electrode sensor.

FIG. 3B is an illustration of the two-electrode sensor shown in FIG. 3A.

FIG. 3C is a schematic of a system of detecting heavy metals in waterwith the two-electrode sensor shown in FIGS. 3A and 3B. The sensor wasimmersed in 100 ml test solution and connected with two AAA batteriesand a 100 kΩ resistor. Voltage across the resistor, ΔV, was measured asthe sensor output.

FIG. 4A is an illustration of a four-electrode sensor geometry accordingto various aspects of the current technology.

FIG. 4B is a schematic of a system of detecting heavy metals in waterwith the four-electrode sensor shown in FIG. 4A. The sensor was immersedin 100 ml test solution and connected as aA-Bb when it was operated.Voltage across the resistor was measured as ΔV₁ when connected as aA-BB′and as ΔV₂ when A′A-Bb.

FIG. 5A is a reading of ΔV from a two-electrode sensor with 5 μm gap.

FIG. 5B is a reading of ΔV from a two-electrode sensor with 10 μm gap.

FIG. 5C shows ΔV readings from a two-electrode sensor having a 5 μm gapin various simulated solutions.

FIG. 6 is an illustration showing that in a two-electrode systemaccording to various aspects of the current technology, where metalsreduce or oxidize into conductive species (drawn as arrows). Somenonconductive salts and rust (drawn as circles) also precipitate on thesensor.

FIG. 7 shows photographs of an exemplary two-electrode sensor (top left)before and after it is operated in various solutions for two weeks. Leaddeposits on an anode while all other metals deposit or precipitate onthe cathode.

FIG. 8 is an illustration showing that in a four-electrode system, leadis oxidized to conductive lead dioxide on an anode, and other metals arereduced to conductive species (drawn as arrows) on a cathode.Nonconductive salts and rust (drawn as circles) also precipitate on thecathode.

FIG. 9A shows an original ΔV₁ reading at an anode of an exemplaryfour-electrode sensor.

FIG. 9B shows an original ΔV₂ reading at a cathode of an exemplaryfour-electrode sensor disposed in different solutions for two weeks.

FIG. 10A shows Auger electron spectra of standard metal lead, standardlead (II) oxide, sample Pb02 (150 ppb Pb) and Pb002 (15 ppb Pb). Theinset expands the Pb N00 Auger transitions in a first order derivative.

FIG. 10B shows Auger electron spectra profiles of an anode and a cathodeof a sample having “Pb 15 ppb+Cu 1 mg/L+Zn 5 mg/L.”

FIG. 11A is a scanning electron microscopy image and electron mapping atPb transition peaks from a Pb02 sample under an electronic impact of 10kV and 10 nA.

FIG. 11B is a scanning electron microscopy image and electron mapping atNOO transition peaks from a Pb02 sample under an electronic impact of 10kV and 10 nA.

FIG. 11C is a scanning electron microscopy image and electron mapping atMNV transition peaks from a Pb02 sample under an electronic impact of 10kV and 10 nA.

FIG. 12A shows an original ΔV₁ reading at an anode of an exemplaryfour-electrode sensor in Tap 1, Tap 2, and Simultap for four weeks.

FIG. 12B shows an original ΔV₂ reading at a cathode of an exemplaryfour-electrode sensor in Tap 1, Tap 2, and Simultap for four weeks.

FIG. 13A shows an anode side of an exemplary four-electrode sensorstored in Tap 1, 15 ppb, and 150 ppb solutions for two weeks. Thefour-electrode sensor functioned normally after the two weeks.

FIG. 13B shows a cathode side of an exemplary four-electrode sensorstored in Tap 1, 15 ppb, and 150 ppb solutions for two weeks. Thefour-electrode sensor functioned normally after the two weeks.

FIG. 13C shows photographs of an exemplary four-electrode sensor. Thephotographs show that hardness precipitated on a cathode side after thesensor was turned on for two weeks, but mostly dissolved again after thesensor was off for another two weeks.

FIG. 13D shows a first exemplary approach for using a four-electrodesensor for long-term monitoring. Here, multiple sensors are in a singlewater pipe.

FIG. 13E shows a second exemplary approach for using a four-electrodesensor for long-term monitoring. Here, two electrodes are being usedalternatively.

Corresponding reference numerals indicate corresponding parts throughoutthe several views of the drawings.

DETAILED DESCRIPTION

Example embodiments are provided so that this disclosure will bethorough, and will fully convey the scope to those who are skilled inthe art. Numerous specific details are set forth such as examples ofspecific compositions, components, devices, and methods, to provide athorough understanding of embodiments of the present disclosure. It willbe apparent to those skilled in the art that specific details need notbe employed, that example embodiments may be embodied in many differentforms and that neither should be construed to limit the scope of thedisclosure. In some example embodiments, well-known processes,well-known device structures, and well-known technologies are notdescribed in detail.

The terminology used herein is for the purpose of describing particularexample embodiments only and is not intended to be limiting. As usedherein, the singular forms “a,” “an,” and “the” may be intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. The terms “comprises,” “comprising,” “including,” and“having,” are inclusive and therefore specify the presence of statedfeatures, elements, compositions, steps, integers, operations, and/orcomponents, but do not preclude the presence or addition of one or moreother features, integers, steps, operations, elements, components,and/or groups thereof. Although the open-ended term “comprising,” is tobe understood as a non-restrictive term used to describe and claimvarious embodiments set forth herein, in certain aspects, the term mayalternatively be understood to instead be a more limiting andrestrictive term, such as “consisting of” or “consisting essentiallyof.” Thus, for any given embodiment reciting compositions, materials,components, elements, features, integers, operations, and/or processsteps, the present disclosure also specifically includes embodimentsconsisting of, or consisting essentially of, such recited compositions,materials, components, elements, features, integers, operations, and/orprocess steps. In the case of “consisting of,” the alternativeembodiment excludes any additional compositions, materials, components,elements, features, integers, operations, and/or process steps, while inthe case of “consisting essentially of,” any additional compositions,materials, components, elements, features, integers, operations, and/orprocess steps that materially affect the basic and novel characteristicsare excluded from such an embodiment, but any compositions, materials,components, elements, features, integers, operations, and/or processsteps that do not materially affect the basic and novel characteristicscan be included in the embodiment.

Any method steps, processes, and operations described herein are not tobe construed as necessarily requiring their performance in theparticular order discussed or illustrated, unless specificallyidentified as an order of performance. It is also to be understood thatadditional or alternative steps may be employed, unless otherwiseindicated.

When a component, element, or layer is referred to as being “on,”“engaged to,” “connected to,” or “coupled to” another element or layer,it may be directly on, engaged, connected or coupled to the othercomponent, element, or layer, or intervening elements or layers may bepresent. In contrast, when an element is referred to as being “directlyon,” “directly engaged to,” “directly connected to,” or “directlycoupled to” another element or layer, there may be no interveningelements or layers present. Other words used to describe therelationship between elements should be interpreted in a like fashion(e.g., “between” versus “directly between,” “adjacent” versus “directlyadjacent,” etc.). As used herein, the term “and/or” includes any and allcombinations of one or more of the associated listed items.

Spatially or temporally relative terms, such as “before,” “after,”“inner,” “outer,” “beneath,” “below,” “lower,” “above,” “upper,” and thelike, may be used herein for ease of description to describe one elementor feature's relationship to another element(s) or feature(s) asillustrated in the figures. Spatially or temporally relative terms maybe intended to encompass different orientations of the device or systemin use or operation in addition to the orientation depicted in thefigures.

Throughout this disclosure, the numerical values represent approximatemeasures or limits to ranges to encompass minor deviations from thegiven values and embodiments having about the value mentioned as well asthose having exactly the value mentioned. All numerical values ofparameters (e.g., of quantities or conditions) in this specification,including the appended claims, are to be understood as being modified inall instances by the term “about” whether or not “about” actuallyappears before the numerical value. “About” indicates that the statednumerical value allows some slight imprecision (with some approach toexactness in the value; approximately or reasonably close to the value;nearly). If the imprecision provided by “about” is not otherwiseunderstood in the art with this ordinary meaning, then “about” as usedherein indicates at least variations that may arise from ordinarymethods of measuring and using such parameters.

In addition, disclosure of ranges includes disclosure of all values andfurther divided ranges within the entire range, including endpoints andsub-ranges given for the ranges. As referred to herein, ranges are,unless specified otherwise, inclusive of endpoints and includedisclosure of all distinct values and further divided ranges within theentire range. Thus, for example, a range of “from A to B” or “from aboutA to about B” is inclusive of A and of B.

Example embodiments will now be described more fully with reference tothe accompanying drawings.

The sensor for detecting metal, such as lead, in water advantageouslyhas a long lifetime so that the sensor can be inserted into a water pipefor years until lead leakage happens. Advantageously, the sensor canautomatically inform users, without regular examination. Sensor that areaffordable would enable most families to have one installed, forexample, at each end point of their water service lines. Accordingly,the current technology provides sensors that can be made in efficientand inexpensive processes that are only about the size of a rice grain(less than or equal to about 1 mm³, not including a power source). Thesmall size of the sensors allows them to be inserted in pipes, and theyrequire only simple circuits and, readily available power sources, forexample, two AAA batteries for operation by way of anon-limitingexample. The sensors may be made with inert platinum electrodes and aresuitable for long-term water monitoring of heavy metals. None-limitingexamples of heavy metals include heavy metals include lead (Pb), zinc(Zn), copper (Cu), iron (Fe), antimony (Sb), arsenic (As), cadmium (Cd),chromium (Cr), mercury (Hg), nickel (Ni), selenium (Se), thallium (Ti),silver (Ag), manganese (Mn), barium (Ba), and combinations thereof.

FIG. 1 shows a two-electrode sensor 10 according to various aspects ofthe current technology. The two-electrode sensor 10 comprises a firstelectrode 12 and a second electrode 14. In various embodiments, thefour-electrode sensor 100 is free of a reference electrode or a ligand.The first electrode 12 comprises a first surface 16 that defines a firstpattern and the second electrode 14 comprises a second surface 18 thatdefines a second pattern. The first and second patterns arecomplementary, i.e., as mirror images or negatives, to each other, suchthey fit together, leaving a gap or path therebetween. For example, thefirst surface 16 and the second surface 18 are separated from each otherby a gap having a distance of greater than or equal to about 500 nm toless than or equal to about 10 μm, greater than or equal to about 750 nmto less than or equal to about 8 μm, or greater than or equal to about 1urn to less than or equal to about 5 μm. The gap distance issubstantially constant, i.e., deviates by less than about 20% of anaverage distance. In some embodiments, the distance is about 1 μm, about2 μm, about 3 μm, about 4 μm, about 5 μm, about 6 μm, about 7 μm, about8 μm, about 9 μm, or about 10 μm.

The designs of the first and second patterns are not limited, other thanthat they are complementary to each other. For example, the firstpattern can comprise at last one peak and at least one valley, whereinthe at least one peak and the at least one valley are individuallysquared, flat, curved, or pointed. Accordingly, in various aspects ofthe current technology, the first and second patterns comprise aplurality of complementary complex peaks and valleys. Put another way,the first and second electrodes 12, 14 have complementary interdigitated(and complex) surfaces that are separated from each other by the gap.

The first electrode 12 is a positive electrode and the second electrode14 is a negative electrode, and each electrode 12, 14 comprises aconductive metal. However, it is understood that the charge of theelectrodes 12, 14 can be switched. Non-limiting examples of conductivemetals include platinum, gold, silver, copper, and combinations thereof.The first and second electrodes 12, 14 have a length Li of greater thanor equal to about 500 μm to less than or equal to about 2 mm, a width Wiof greater than or equal to about 50 μm to less than or equal to about 1mm, and a thickness T₁ of greater than or equal to about 250 Å to lessthan or equal to about 2000 Å. Moreover, the first electrode 12 has asurface area of greater than or equal to about 0.4 mm² to less than orequal to about 0.5 mm² and the second electrode 14 has a surface area ofgreater than or equal to about 0.3 mm² to less than or equal to about0.4 mm². The first and second electrodes 12, 14 are also characterizedby a contact length to surface area ratio of from greater than or equalto about 5 cm⁻¹ to less than or equal to about 20 cm⁻¹, wherein thecontact length is the distance between the electrodes. However, it isunderstood that the dimensions of the first and second electrodes 12, 14can be scaled up or scaled down, depending on conditions in which thetwo-electrode sensor 10 will be used. For example, the dimensions may bescaled down when the two-electrode sensor 10 is inserted into a smallwater pipe or scaled up when the two-electrode sensor 10 is insertedinto a large water pipe.

As shown in FIG. 1, the first and second electrode 12, 14 are optionallycoupled to a substrate 20 by way of an adhesive layer 22 disposedbetween the first and second electrodes 12, 14 and the substrate 20. Theoptional adhesive layer 22 has a thickness T₂ of greater than or equalto about 50 nm to less than or equal to about 500 nm. The substrate iscrystalline or amorphous and comprises silicon dioxide (glass) or anyother material known in the art. The adhesive layer 22 comprises, asnon-limiting examples, titanium, chromium, or a combination thereof.Additionally, the first and second electrodes 12, 14 are optionallycoupled to a solid support 24. The solid support 24 comprises anon-conductive material, such as, for example, a polymer, such as aplastic, a glass, or a printed circuit (PC) board.

The first electrode 12 and the second electrode 14 are connected to apower source 26, for example, by a first lead 28 and a second lead 30,respectively. The first and second leads 28, 30 are wires, circuitsprinted on a circuit board, or a combination thereof. The power source26 is not limited, and can be, for example, a battery, a plurality ofbatteries, a photovoltaic device, or an electrical service of abuilding, such as a home.

The two-electrode sensor 10 is configured to detect heavy metals inwater without incorporating a reference electrode or a ligand. Forexample, when the two-electrode sensor 10 contact water comprising heavymetals, such as lead (Pb), zinc (Zn), copper (Cu), iron (Fe), antimony(Sb), arsenic (As), cadmium (Cd), chromium (Cr), mercury (Hg), nickel(Ni), selenium (Se), thallium (T₁), silver (Ag), manganese (Mn), barium(Ba), and combinations thereof, as non-limiting examples, and when anelectric potential is applied between the first and second electrodes12, 14, lead is oxidized in lead dioxide and deposited at the firstelectrode 12 and the other metals are reduced and deposited at thesecond electrode 14. A change in voltage relative to a baseline valueobtained in the absence of detectable heavy metals, indicates thepresence of heavy metals. The two-electrode sensor 10 indirectlyquantifies a heavy metal concentration in that the shorter the timebetween operating the sensor and recording a voltage change, i.e., theshorting of the electrodes, the higher the concentration of the heavymetal.

FIG. 2 shows a four-electrode sensor 100 according to various aspects ofthe current technology. The four-electrode sensor 100 is similar to thetwo-electrode sensor 10 shown in FIG. 1, but with a pair of first andsecond electrodes 12, 14. More particularly, the four-electrode sensor100 comprises a first electrode 102, a second electrode 104, a thirdelectrode 106, and a fourth electrode 108. In various embodiments, thefour-electrode sensor 100 is free of a reference electrode and a ligand.The first electrode 102 comprises a first surface 110 that defines afirst pattern, the second electrode 104 comprises a second surface 112that defines a second pattern, the third electrode 106 comprises a thirdsurface 114 that defines a third pattern, and the fourth electrode 108comprises a fourth surface 116 that defines a fourth pattern. The firstand second patterns, and the third and fourth patterns, arecomplementary, i.e., as mirror images or negatives, to each other, suchthey fit together, leaving a gap or path therebetween. For example, thefirst surface 110 and the second surface 112, and the third surface 114and the fourth surface 116, are separated from each other by individualgaps having a distance of greater than or equal to about 500 nm to lessthan or equal to about 10 μm, greater than or equal to about 750 nm toless than or equal to about 8 μm, or greater than or equal to about 1 μmto less than or equal to about 5 μm. The gap distance is substantiallyconstant, i.e., deviates by less than about 20% of an average distance.In some embodiments, the gap distance is about 1 μm, about 2 μm, about 3μm, about 4 μm, about 5 μm, about 6 μm, about 7 μm, about 8 μm, about 9μm, or about 10 μm. Also, the second electrode 104 and the thirdelectrode 106 are separated from each other by a distance of greaterthan or equal to about 1 μm to less than or equal to about 500 μm, suchas by a distance of about 10 μm, about 20 μm, about 30 μm, about 40 μm,about 50 μm, about 60 μm, about 70 μm, about 80 μm, about 90 μm, about100 μm, about 250 μm, or about 500 μm.

The designs of the first, second, third and fourth patterns are notlimited, other than that the first and second patterns are complementaryto each other and the third and fourth patterns are complementary toeach other. For example, the first or third pattern can comprise at lastone peak and at least one valley, wherein the at least one peak and theat least one valley are individually squared, flat, curved or pointed.Accordingly, in various aspects of the current technology, the first andsecond patterns comprise a plurality of complementary complex peaks andvalleys and the third and fourth patterns comprise a plurality ofcomplementary peaks and valleys. Put another way, the first and secondelectrodes 102, 104 have complementary interdigitated surfaces 110, 112that are separated from each other by the first gap and the third andfourth electrodes 106, 108 have complementary (and complex)interdigitated surfaces 114, 116 that are separated from each other bythe second gap.

In electroplating operation mode, the first electrode 102 is a positiveelectrode and the fourth electrode 108 is a negative electrode. However,it is understood that the charge of the electrodes 102, 108 can beswitched. Each of the first, second, third, and fourth electrodes 102,104, 106, 108 comprises a conductive metal as described above in regardto FIG. 1. The first, second, third, and fourth electrodes 102, 104,106, 108 have a length Li of greater than or equal to about 500 μm toless than or equal to about 2 mm, a width Wi of greater than or equal toabout 50 μm to less than or equal to about 1 mm, and a thickness T₁ ofgreater than or equal to about 250 Å to less than or equal to about 2000Å. Moreover, the first electrode 102 has a surface area of greater thanor equal to about 0.4 mm² to less than or equal to about 0.5 mm², thesecond electrode 104 has a surface area of greater than or equal toabout 0.4 mm² to less than or equal to about 0.5 mm², the thirdelectrode 106 has a surface area of greater than or equal to about 0.1mm² to less than or equal to about 0.3 mm², and the fourth electrode 108has a surface area of greater than or equal to about 0.3 mm² to lessthan or equal to about 0.4 mm². The first, second, third, and fourthelectrodes 102, 104, 106, 108 are also characterized by a contact lengthto surface area ratio of from greater than or equal to about 5 cm⁻¹ toless than or equal to about 20 cm⁻¹. However, it is understood that thedimensions of the first, second, third, and fourth electrodes 102, 104,106, 108 can be scaled up or scaled down, depending on conditions inwhich the four-electrode sensor 100 will be used. For example, thedimensions may be scaled down when the four-electrode sensor 100 isinserted into a small water pipe or scaled up when the four-electrodesensor 100 is inserted into a large water pipe.

As shown in FIG. 2, the first, second, third, and fourth electrodes 102,104, 106, 108 are optionally coupled to a substrate 118 by way of anadhesive layer 120 disposed between the first, second, third, and fourthelectrodes 102, 104, 106, 108 and the substrate 118. The optionaladhesive layer 120 has a thickness T₂ of greater than or equal to about50 nm to less than or equal to about 500 nm. The substrate 118 iscrystalline or amorphous and comprises silicon dioxide (glass) or anyother material known in the art. The adhesive layer 120 comprises, asnon-limiting examples, titanium, chromium, or a combination thereof.Additionally, the first, second, third, and fourth electrodes 102, 104,106, 108 are optionally coupled to a solid support 122. The solidsupport 122 comprises a non-conductive material, such as, for example, apolymer, such as a plastic, a glass, or a printed circuit (PC) board.

The first, second, third, and fourth electrodes 102, 104, 106, 108 areelectrically connected to a first lead 126, a second lead 128, a thirdlead 130, and a fourth lead 132, respectively. Moreover, the first,second, third, and fourth leads 126, 128, 130, 132 are independently andindividually connectable to a power source 124. Put another way, thesensor 100 is configured such that the first, second, third, and fourthleads 126, 128, 130, 132 can be individually coupled to and decoupledfrom the power source 124. The first, second, third, and fourth leads28, 30 are wires, circuits printed on a circuit board, or a combinationthereof. The power source 124 is not limited, and can be, for example, abattery, a plurality of batteries, a photovoltaic device, or anelectrical service of a building, such as a home. The power source 124has a connectable end A and a second connectable end B. The first lead126 has a connectable end a, the second lead 128 has a connectable endB′, the third lead has a connectable end A′, and the fourth lead 132 hasa connectable end b. Each of the connectable ends a, B′, A′, and b canbe individually and reversibly electronically connected to connectedends A and B of the battery 124. For example, when an electric potentialis applied the first and fourth electrodes 102, 108 via an aA-Bbconnection, heavy metals are electroplated on either the second or thirdelectrode 104, 106 depending on the standard reduction potential ofindividual heavy metals.

The four-electrode sensor 100 is configured to selectively detect leadin water without incorporating a reference electrode or a ligand. Forexample, the four-electrode sensor 100 is placed in an environmentwherein it contacts water comprising heavy metals, such as lead (Pb),zinc (Zn), copper (Cu), iron (Fe), antimony (Sb), arsenic (As), cadmium(Cd), chromium (Cr), mercury (Hg), nickel (Ni), selenium (Se), thallium(Ti), silver (Ag), manganese (Mn), barium (Ba), and combinationsthereof, as non-limiting examples. When the electrode 100 is connectedin an aA-Bb configuration, an electric potential (sufficient to reducelead ions to lead oxide) is applied between the first and fourthelectrodes 102, 108, lead is oxidized into lead dioxide, and the leaddioxide is electroplated onto the second electrode 104. In variousembodiments, 1.5 V is applied. Meanwhile the remaining heavy metals arereduced and electroplated onto the third electrode 106. When theelectrode is then connected in an aA-BB′ configuration, an electricpotential is applied between the first and second electrodes 102, 104. Achange in voltage relative to a baseline measurement when the electrode100 is disposed in water that does not contain detectable levels oflead, signifies that lead is present in the water. When the electrode isthen connected in an A′A-Bb configuration, an electric potential isapplied between the third and fourth electrodes 106, 108. A change involtage relative to a baseline measurement when the electrode 100 isdisposed in water that does not contain detectable levels of heavymetals signifies that heavy metals other than lead are present in thewater. Although not shown in FIG. 2, in various embodiments thefour-electrode sensor 100 comprises an alert feature that provides atleast one of an audible and visual alert when at least one of lead oranother heaving metal is detected in water. The four-electrode sensor100 indirectly quantifies a heavy metal concentration in that theshorter the time between operating the sensor and recording a voltagechange, i.e., the shorting of the electrodes, the higher theconcentration of the heavy metal.

Accordingly, the current technology also provides as water pipe havingan internal bore section through which water flows, wherein thefour-electrode sensor 100 is disposed within the internal bore section.

The current technology further provides a method for fabricating thetwo-electrode sensor 10 shown in FIG. 1 or the four-electrode sensor 100shown in FIG. 2. The method comprises disposing an adhesive layer on asubstrate; disposing a photoresist onto the adhesive layer; anddisposing a photoresist mask on the photoresist. The photoresist maskcomprises a pattern defining either the first and second electrode 12,14 of the two-electrode sensor 10 of FIG. 1 or the first, second, third,and fourth electrodes 102, 104, 106, 108 of the four-electrode sensor100 of FIG. 2. As a non-limiting example, the pattern can define a firstelectrode and a second electrode, the first electrode and the secondelectrode having complementary surfaces that are separated from eachother by a distance of greater than or equal to about 500 nm to lessthan or equal to about 10 μm; a third electrode and a fourth electrode,the third electrode and the fourth electrode having complementarysurfaces that are separated from each other by a distance of greaterthan or equal to about 500 nm to less than or equal to about 10 μm;wherein the second electrode and the third electrode are separated fromeach other by a distance of greater than or equal to about 1 μm to lessthan or equal to about 1 mm.

The method then comprises transferring the pattern of the photoresistmask into the adhesive layer to generate a patterned adhesive layer; anddisposing a layer of a conductive material onto the patterned adhesivelayer. Each of the substrate, the adhesive layer, and the conductivematerial are described above.

The current technology also provides a method for continuouslymonitoring a water sample for the presence of heavy metals. In certainvariations, heavy metals include, e.g., in the water sample, lead (Pb),zinc (Zn), copper (Cu), iron (Fe), antimony (Sb), arsenic (As), cadmium(Cd), chromium (Cr), mercury (Hg), nickel (Ni), selenium (Se), thallium(Ti), silver (Ag), manganese (Mn), barium (Ba), and combinationsthereof. In certain preferred aspects, the heavy metal is lead (Pb). Themethod comprises contacting a sensor with the water sample. The watersample can be contained in a vessel. The vessel is non-limited and canbe, for example, a pipe or a container, such as a glass or a pitcher.The pipe can be, for example, a water pipe in a building, such as ahouse, apartment, condominium, office, or commercial building. Thesensor can be any sensor described above. In various aspects of thecurrent technology, the sensor comprises a first electrode and a secondelectrode, the first electrode and the second electrode havingcomplementary interdigitated surfaces that are separated from each otherby a first gap having a first distance of greater than or equal to about500 nm to less than or equal to about 10 μm, and a third electrode and afourth electrode, the third electrode and the fourth electrode havingcomplimentary complementary interdigitated surfaces that are separatedfrom each other by a second gap having a second distance of greater thanor equal to about 500 nm to less than or equal to about 10 μm. Thesecond electrode and the third electrode are separated from each otherby a distance of greater than or equal to about 1 μm to less than orequal to about 1 mm. In various embodiments, the sensor is thefour-electrode sensor 100 described in FIG. 2. In various embodiments,the method is free of using a reference electrode or ligands.

The method also comprises applying a first electrical potential betweenthe first and fourth electrodes. The first electrical potential causesthe oxidation of lead (Pb) to lead dioxide (PbO₂), which electroplateson the second electrode. The first electrical potential also causes thereduction of other heavy metals, which electroplate on the thirdelectrode.

The method then comprises applying a second electrical potential betweenthe first and second electrodes, and measuring a first voltage betweenthe first and second electrodes. The first voltage is compared to abaseline voltage in water that does not contain detectable levels oflead. Therefore, the method comprises determining that lead is presentin the water when the first voltage is different from a baseline voltagein water that does not contain detectable levels of lead. In someembodiments, the method includes generating an alert when the firstvoltage is different from a baseline voltage in water that does notcontain detectable levels of lead. The alert can be at least one of anaudible and a visual alert.

In some embodiments, the method further comprises, after the measuring afirst voltage between the first and second electrodes, applying a thirdelectrical potential between the third and fourth electrodes, andmeasuring a second voltage between the third and fourth electrodes. Thesecond voltage is compared to a baseline voltage in water that does notcontain detectable levels of heavy metals other than lead. Therefore,the method comprises determining that heavy metals other than lead arepresent in the water when the second voltage is different from abaseline voltage in water that does not contain detectable levels heavymetals other than lead. In some embodiments, the method includesgenerating an alert when the second voltage is different from a baselinevoltage in water that does not contain detectable levels of heavymetals. The alert can be at least one of an audible and a visual alert.

Embodiments of the present technology are further illustrated throughthe following non-limiting examples.

Example 1

Leakage of lead and other heavy metals into drinking water is asignificant health risk and one that is not easily detected. Simplesensors containing only platinum electrodes for the detection of heavymetal contamination in drinking water are now described. A two-electrodesensor can identify the existence of a variety of heavy metals indrinking water, and a four-electrode sensor can distinguish lead fromother heavy metals in solution. No false-positive response is generatedwhen the sensors are placed in simulated and actual tap watercontaminated by heavy metals. Lead detection on the four-electrodesensor is not affected by the presence of common ions in tap water.Experimental results suggest that the sensors can be embedded in waterservice lines for long periods of time until lead or other heavy metalsare detected. With its low cost (˜$0.10/sensor) and long-term operation,the sensors are ideal for heavy metal detection of drinking water.

Methods

Fabrication of Electrodes.

Sensors (FIGS. 3A-3B) is constructed using physical vapor deposition of300/1000 Å Ti/Pt on a 500 μm thick, 4 inch diameter glass wafer.Pressure is controlled under 2×10⁻⁶ Torr with a deposition rate of 15and 5 Å/s, respectively. The sensors are integrated with a PC board asshown in FIG. 3A. A two-electrode system is shown in FIG. 3B, and theelectrodes are separated with a 5 or 10 μm gap. A four-electrode sensoris fabricated by the same method but in a different geometry as shown inFIG. 4A. Small gaps between left two electrodes and right two electrodesare 5 μm, and a large gap between the middle two electrodes is 50 μm.

Experiment Setup and the Measurement of the Impedances.

In FIG. 3C, an integrated two-electrode sensor is connected with two AAAbatteries and a 100 kΩ resistor. The sensor is dipped in 100 ml testsolution in a beaker. The voltage difference across a resistor, ΔV, ismeasured by Labview as the signal. AV reflects the overall impedanceacross the electrodes: ΔV increases when the impedance across the twoelectrodes decreases. All solutions are changed every week during theexperiments. The schematic diagram of the four-electrode system is shownin FIG. 4B. The sensor is connected with two AAA batteries and a 100 kΩresistance. When the sensor is operated and electroplating metals, thesensor is connected as aA-Bb. Voltage difference across the resistor ismeasured as ΔV₁ when the sensor is reconnected as aA-BB′ to measureimpedance between the anode and the second electrode. When the sensor isreconnected as A′A-Bb, the voltage difference across the resistor ismeasured as ΔV₂ to detect the impedance between the cathode and thethird electrode.

Test Solutions.

Simulated test solutions are made with PbCl₂, CuCl₂, ZnCl₂, and FeCl₂ in10⁻² M NaCl made with DI water. The NaCl is added to increase theconductivity of the solution to about 1000 μS/cm, the upper limit ofdrinking water set by EPA. The composition of the simulated tap water(Simultap) and Ann Arbor tap water (information gathered from annual AnnArbor water quality reports 2003-2015) is listed in Table 1. Simultapcontains relatively higher concentrations of common ions relative toreal tap water. The real sample Tap 1 and Tap 2 are collected in AnnArbor, Mich., USA. The heavy metals in the real tap water samples areexamined with ICPMS and listed in Table 2. The lead concentration istested both by ICPMS in a University of Michigan and National testinglaboratory approved by the EPA. Tap 1 contains no lead and relativelylow concentration of all heavy metals. Tap 2 contains about 5 ppb oflead, which is smaller than action level (15 ppb), and 0.7 mg/L ofcopper, which is relatively high but smaller than SMCL. PbCl₂ is addedin Tap 1 to make the “Tap 1+Pb 150 ppb” sample but no extra NaCl wasadded.

TABLE 1 Ion concentration in simulated tap water (Simultap) and real AnnArbor, MI tap water. Ion Simultap (mg/L) Ann Arbor (mg/L) Na⁺ 270 48-67K⁺ 11 — Mg²⁺ 71 10-33 Ca²⁺ 46 23-66 HCO₃ ⁻ 61 — CO₃ ²⁻ 14 100-176 NO₃ ⁻18   0-0.06 SO₄ ²⁻ 390 41-82 Cl⁻ 364  98-147

TABLE 2 Concentration of heavy metals ions in tap water samples and EPAregulation. EPA Metal Unit Tap 1 Tap 2 regulations Notes Pb ppb ND 3.0 0(MCL); NL: National Testing (NL) 15 (AL) laboratory Pb ppb ND 5.0 0(MCL); ND: Not detectable 15 (AL) (<1 ppb) Cu mg/L 0.004 0.70 1.3 (MCL);MCL: Maximum 1.0 (SMCL) contaminant level Zn mg/L 0.004 0.59 5.0 SMCL:Secondary (SMCL) maximum contaminant level Fe mg/L 0.003 0.033 0.3(SMCL) AL: Action Level Al mg/L 0.024 0.012 0.050-0.2 (SMCL) Cr mg/L0.0002 0.0004 0.1 (SMCL) Mn mg/L 0.0001 0.005 0.05 (SMCL)

Auger Spectroscopy.

Auger spectroscopy data is collected for the specimens on a PHI 680Auger nanoprobe that is equipped with a field emission electron gun anda cylindrical mirror energy analyzer (energy resolution ΔE/E≈0.25%). Abase pressure of the test chamber is about 1.2×10{circumflex over ( )}-9torr. The native oxidized layer of the chromium pellet is removed by Arion sputtering. To avoid the charging effect of insulating samples underelectron beam irradiation, Pb oxides powder with size less than 3 μm ispressed into a tin foil or a carbon type so that a high energy electronbeam can penetrate these lead oxide particles, while “devices” areplaced on the tilt stage in order to reduce the embedded charging effectcaused by the deep penetration of incident electron beams. A smallelectron beam current of 1 nA is used to irradiate the specimens.

Results and Discussion

Simple sensors for detecting heavy metal in drinking water are achievedwith simple platinum electrodes. When the electrodes are connected with2 AAA batteries (˜3.2V), heavy metal ions are reduced to conductivemetals on the cathode. As shown in Table 3, the electric resistances ofreduced metals are 9 to 10 orders of magnitude smaller than drinkingwater. Thus, when reduced metals connect the gap between the electrodes,the impedance across the electrodes drops significantly. The impedancechange is an indicator of the existence of heavy metals in the water.

TABLE 3 Resistivity of reduced and oxidized metals and drinking water.Resistivity Reduced Resistivity Oxidized metal (Ω · m) metal (Ω · m)PbO₂ 2-74 × 10⁻⁶ Pb 2.20 × 10⁻⁷ ZnO >2.2 Zn 5.90 × 10⁻⁸ CuO 25-100  Cu1.68 × 10⁻⁸ Cu₂O 10²-10⁴  Fe(OH)₃/FeO(OH)/ 10³-10⁶  Fe 1.00 × 10⁻⁷Fe(OH)₂/Fe₂O₃ Drinking water 10-2000 — —

Two-Electrodes System

A two-electrode sensor with 5 μm gaps can detect lead ions at a level of15 ppb with no false responses. The performance of the 5 μm gap,two-electrodes system is shown in FIG. 5A and a 10 μm gap sensor in FIG.5B. For both sensors, ΔV increases significantly and becomes greaterthan 1V within two days in 150 ppb Pb²⁺ solution. The growth of ΔVrepresents conductive layers formed between the two electrodes; thus,reducing the impedance. The sensor with a 5 μm gap shows a response (ΔVgreater than 1V) in 15 ppb Pb²⁺ (action level) solution in three days,but the sensor with 10 μm gap shows no response throughout the two weeksexperiment. These results suggest that a 5 μm gap between the electrodesis more sensitive than a larger gap. Both sensors have no false positiveresponse from Simultap, showing common ions in water did not generateconductive species.

The two-electrodes sensor with 5 μm gap shows a response to almost allsolutions with heavy metals and shows no false positive responses. Thesensor is tested in various simulated solutions designed to mimic theEPA heavy metal regulations listed in Table 2, and the performance isplotted in FIG. 5C. AV increases in all heavy metal solutions butremains the same in Simultap, which contains no heavy metal ions. Thevariation of ΔV is because some conductive deposition may fall offduring the two-week experiment, and the time ΔV remains greater than 1Vis not crucial. The sensor is also tested in two real tap water samplesand a mixture of real tap water and lead. The performance in both thesimulated and real samples is shown in Table 4. The solutions with leadhigher than the action level, and solutions with no heavy metal ions,are listed. The sensor shows fast response (<3 days) to lead, zinc, andcopper solutions. No false positive response was generated in eitherSimultap or Tap 1.

TABLE 4 Two-electrode sensor performance in different solutions.Response day Max ΔV Solution (days) (V) Simulated Pb 150 ppb 3 2.25sample Pb 15 ppb 3 1.89 Fe 6.0 mg/L 9 1.00 Fe 0.3 mg/L 13 1.01 Zn 5.0mg/L 2 1.25 Zn 0.5 mg/L 3 1.27 Cu 1.0 mg/L 2 3.15 Cu 0.1 mg/L 1 3.20Simultap — 0.4 Real Tap 1 — 0.8 sample Tap 1 + Pb 1 1.65 150 ppb Tap 1 +Pb — 0.60 15 ppb Tap 2 8 1.60

However, the responses in simulated ferrous solution and the mixture ofreal tap water and lead ions are slower than expected. The sensorgenerates a slow and weak response (max ΔV=1V at 9th day) in simulatedferrous solutions even at very high concentrations (20 times larger thanSMCL). For the same 15 ppb Pb concentration, the sensor responds in 3days in 15 ppb Pb solution but does not respond to the mixture of Tap1+15 ppb Pb. The sensor also responds more slowly to Tap 2 (0.7 mg/L Cu)than 0.1 mg/L Cu solution.

Operation of the Sensor

The performance of the sensor can be explained with the help of FIG. 6.The sensor shows a response only if conductive deposition connects thegap and thus decreases the impedance between electrodes. The tendency ofmetal ions reducing to conductive metal can be represented by thestandard reduction potentials, E⁰. The higher the E⁰, the easier theions can be reduced. E⁰ values of common metal ions in contaminateddrinking water are listed in Table 5. E⁰ _(acid) is the E⁰ in acid(pH=0) and E⁰ basic is the value in basic (pH=14) conditions. Pb²⁺,Zn²⁺, Fe²⁺, and Cu²⁺ can be reduced to conductive metals when thepotential on the cathode is smaller than −0.76. With 2 AAA batteries,the potential on the cathode is sufficient to reduce the heavy metalions.

TABLE 5 Standard potential E⁰ of metal ions in drinking water. ReactionE⁰acid (V) E⁰basic (V) PbO₂/Pb²⁺ 1.46 — O₂/H₂O 1.23 — Pt₂+/Pt 1.18 —Fe³⁺/Fe²⁺ 0.77 — Cu⁺/Cu 0.52 — Cu²⁺/Cu 0.34 — PbO₂/Pb(OH)₂ — 0.25 H⁺/H₂0.00 −0.83 Pb²⁺/Pb −0.13 — Fe²⁺/Fe −0.44 — Zn²⁺/Zn −0.76 —

Lead ions are the only ions that can deposit a conductive species aroundthe anode. The dominant reaction around the anode is oxidation, and leadis the only element that can be oxidized into a conductive species,i.e., lead dioxide. Generation of lead dioxide is possible because theE⁰ of PbO₂/Pb²⁺ is 1.46V (Table 5). Lead dioxide is consideredconductive because its resistivity is about six to eight orders ofmagnitude smaller than drinking water and the other oxidized metals(Table 3).

No false positive response is possible from typical ions in tap water.

Concentrations of major ions in Ann Arbor, Mich., tap water are listedin Table 1 as an example. Though the concentrations of ions vary fromlocation to location, the species are mostly the same. The standardreduction potentials of these ions are listed in Table 6. Unless thecathode potential is smaller than −2.3V (which is 1.4V smaller than thepotential required to reduce the heavy metals), no conductive speciesare likely to deposit on the sensor surface and drop the impedance. With2 AAA batteries, false positive responses are not likely.

TABLE 6 Standard potential E⁰ of major ions in drinking water. ReactionE⁰ _(acid) (V) E⁰ _(basic) (V) Cl₂/Cl⁻ 1.40 1.36 O₂/H₂O 1.23 — NO₃ ⁻/NO₂0.94 — SO₄ ²⁻/S 0.35 — H⁺/H₂ 0.00 −0.83 NO₃ ⁻/NH₃ — −0.12 CO₃ ²⁻/CH₄ —−0.73 SO₄ ²⁻/SO₃ ²⁻ — −0.94 Mg²⁺/Mg −2.36 — Na⁺/Na −2.72 −2.72 K⁺/K−2.94 −2.94 Ca²⁺/Ca −2.87 —

Though false positive responses are unlikely, the performance of thetwo-electrode sensor may be delayed by precipitated hardness and rust.The solubility of water hardness, which is white with the majorcomponent being calcium carbonate, decreases with increasing pH. With 2AAA batteries (˜3.2V), the sensor electrolyzes water during operation.Thus the local pH around the anode is acidic and basic around thecathode. Hardness precipitates on the cathode, blocking the gap betweenthe electrodes, and delaying the sensor response. Rust, which is mostlyferric and ferrous oxide, is another precipitation that is possible dueto altered pH. Though E⁰ suggests ferric and ferrous ions are possibleto be reduced into iron, previous research shows the ions may insteadprecipitate as rust. The ability of the sensor to detect iron is thuslower than the ability to detect other metals, so the sensor showedweaker and slower response in ferrous solution than in other heavy metalsolutions.

FIG. 7 shows pictures of the sensors operated in different testsolutions and corroborates the hypothesis described above. Lead is theonly element deposited on the anode (+) while zinc and copper arereduced on the cathode (−). Simultap precipitates white hardness, andiron solutions precipitates red rust. Thus, the two-electrode sensor isideal for heavy metal detection but does not distinguish lead from otherheavy metals. Lead is the most toxic metal in drinking water and shouldbe identified for the safety of the users.

Four-Electrode System

To distinguish the most toxic element, lead, from other heavy metals, afour-electrode sensor is designed and tested. Two extra electrodes areplaced between the cathode and the anode as shown in FIGS. 4A and 4B.The small gap between the left two electrodes and the right twoelectrodes is 5 μm. The large gap between the middle two electrodes is50 μm. As explained previously, lead ions are the only ions that willdeposit a conductive species around the anode while other heavy metalscan still deposit on the cathode. The four-electrode sensor thuscontains both a lead detector and a heavy metal sensor.

The expected reactions in the four-electrode system are illustrated inFIG. 8. Lead ions oxidize to lead dioxide around the anode and connectsthe gap between the anode and the second electrode. At the cathode,other metals are reduced, and hardness and rust precipitate due to pHchange. Because lead is the only ion that can be oxidized to aconductive species in the system, lead is the only element that depositsa conductive compound around the anode. The two electrodes on the leftare thus lead detectors and the two electrodes on the right are otherheavy metal sensors.

The concept is confirmed with experiment results and had no falsepositive response on both sides in simulated and real tap water. Theoriginal reading of the four-electrode system are shown in FIGS. 9A and9B. Both ΔV₁ and ΔV₂ maintains less than 1V in Simultap and Tap 1. ΔV₁increases significantly in both Pb²⁺ solutions, and shows no falsepositive response to high concentrations of zinc and iron. ΔV₂ detectsthe existence of all other heavy metals and increases significantly inZn²⁺, Cu²⁺, and Fe²⁺solutions. ΔV₂ does not respond in 15 ppb Pb²⁺because the low ion concentration and most of Pb²⁺ is oxidized on theanode.

One downside is that copper, which is also a toxic metal regulated byEPA MCLs, can generate a false response on the lead detector. A lateresponse (12th day) on ΔV₁ appears in the 1 mg/L copper solution. Thisresponse occurs because copper is the easiest ion to be reduced amongthe four metal ions (Cu²⁺/Cu is 0.34 V as listed in Table 5). On theanode, oxidations are the major reactions and few reductions happen dueto the forced electrical current. However, both oxidation and reductionare possible on the middle two floating electrodes, which means thatcopper could be reduced on these two electrodes as well. When copper isreduced on the second electrode, the anode and the electrode may beconnected. The impedance between these two electrodes dropssignificantly, ΔV₁ increases, and a false positive is generated.

The lead detection using the four-electrode sensor can occur withoutbeing influenced by the main ions in the tap water. Table 7 lists theperformance of the four-electrode sensor in various solutions. Thesolutions with lead levels higher than the action level, and solutionswith no heavy metal ions, are listed. The lead sensor detects allsolutions with lead levels above the action level though the lowconcentration (5 ppb) of lead in Tap 2 is not detected. The ability forlead detection is not influenced by the major ions in the solution.Since the hardness is precipitated around the cathode due to pH changeand it is not blocking the lead detector, the response days for theaction level sample “15 ppb Pb” is the same with the real tap watersample “Tap 1+Pb 15 ppb”. On the other hand, the heavy metal sensor isdelayed by the harness precipitated around the cathode. For the sameconcentration of lead, copper, and zinc, the sensor detects much faster(1 days) in a simulated solution than in the mixture of heavy metals andreal tap water. The heavy metal detector also shows no response to Tap2, which contains a relatively high concentration of copper (0.7 mg/L).

TABLE 7 Four-electrode sensor performance in different solutions. ΔV₂ΔV₁ Anode Cathode ΔV₂ Response ΔV₁ Anode Response Cathode day Max ΔV dayMax ΔV Solution (days) (V) (days) (V) Simulated Pb 150 ppb 1 3.1 5 2.76Pb 15 ppb 7 2.72 — 0.76 Fe 6.0 mg/L — 0.89 10 3.19 Zn 5.0 mg/L — 0.67 33.22 Zn 0.5 mg/L — 0.61 3 3.12 Cu 1.0 mg/L 12 1.86 1 3.13 Cu 0.1 mg/L —0.44 2 3.07 Pb 15ppb + 2 2.99 1 3.21 Cu 1 mg/L + Zn 5 mg/L Simultap —0.31 — 0.59 Real Tap 1 — 0.57 — 0.71 sample Tap 2 — 0.69 — 0.7 Tap 1 +Pb 2 3.05 — 0.56 150 ppb Tap 1 + Pb 7 2.14 12 2.4 15 ppb Tap 1 + Pb 91.36 5 3.02 15 ppb + Cu 1 mg/L + Zn 5 mg/L

Validation with Auger Spectroscopy

The compositions of the metal depositions on the anode and cathode areconfirmed by using Auger electron spectros-copy (AES). AES is asurface-sensitive characterization technique based on the analysis ofenergetic electrons emitted from an excited atom after a series ofinternal relaxation events. The energy position and shape of an Augerpeak contains a significant amount of information about the chemicalenvironment of the source ion. This chemical information results fromthe dependence of the atomic energy levels, the loss structure, and thevalence band structure on the local bonding. Compared to the high andslowly changing backscattered electron background, the Auger peaksusually look small. Commonly, the first order derivatives of the spectraare employed to highlight chemical changes.

The chemical states of the lead deposition on the anode can be validatedby comparing the kinetic energy of the valence band Auger electrons. Inthe experiment, 99.99% Pb and 99.999% PbO are purchased from SigmaAldrich and used as validation standards. Pb02 is the sensor anodeoperated in 150 ppb Pb solution for two weeks and Pb002 is in 15 ppb. Asseen from FIG. 10A (inset), electron beam excited Pb ONN Augertransitions shows a high sensitivity to the chemical states. Themetallic Pb ONN Auger electrons (98.0 eV) have higher kinetic energiesthan those of PbO (87.6 eV) and the specimen (86.5 eV). Similarobservations also occur at Pb MNV transitions (1800-2300 eV) in the rawdata (FIG. 10A). The kinetic energy of the Auger electron depends onlyon the energy levels involved, however, not on the energy of the primaryexcitation. These energy levels relate to the type of atom and thechemical environment in which the atom is located. The energy levels areelement specific, so that the Auger electrons emitted by the samplecarry information about their chemical composition. The resultingspectra are used to determine the identity of the emitting atoms andsome information about their environment. Basically, the inner shellenergy levels are much less affected by the chemical states, so thekinetic energy of the valence band Auger electrons can directly reflectthe chemical states of the source ions.

Another approach to validate the chemical status is the Auger peakintensities, and with the NOO positions it can be concluded that thedeposition on the anode had PbO₂. The Auger peak intensities aredetermined by the ionization cross section, Auger yield possibility, themean escape depth and the backscattering factor. It is quite difficultto individually quantify these factors. Usually, the intensity(peak-to-valley height) of AES peaks can be simplified to the product ofa sensitivity factor and the concentration of the element. Based on thesensitivity factors for 0 and Pb elements derived from the standard PbO,the atomic concentration ratio of the specimens can be calculated fromtheir peak-to-valley heights (see Table 8). By combining the peak Pb NOOposition it can be concluded that the deposition in 15 ppb Pb solutionis mainly PbO₂ while a mixture of PbO₂, PbO, and Pb in 150 ppb solution.

TABLE 8 The averaged atomic ratio obtained from AES data of selectedspots. The errors for all elements are estimated to be 5%. Specimen C OPb Zn Cu Na Cl O/Pb Pb standard — — 100.0 — — PbO standard 31.3 34.434.3 — — 1.0 Pb02 (150 ppb) 15.4 37.9 22.8 — — 11.9 11.8 1.7 Pb002 (15ppb) 55.2 17.2 8.4 — — 10.2 10.1 2.1 Pb 15 ppb + Cu 30.5 45.4 29.1 — — —— 1.6 1 mg/L + Zn 5 mg/L (anode +) Pb 15 ppb + Cu 15.5 34.4 — 40.9 6.8 —— 1 mg/L + Zn 5 mg/L (cathode −)

As seen from FIG. 10B, when the sensor operates in the mixed solution(Pb 15 ppb+Cu 1 mg/L+Zn 5 mg/L), most Pb deposites on the anode while Cuand Zn deposites on the cathode. No Pb (or a trace amount of Pb)deposites on the cathode confirming that the sensor has high elementalselectivity. The atomic ratios are listed in Table 8, which are theaverage of 5 different spots in order to provide the reproducibility.The errors for all elements are estimated to be 5%.

The electron-excited Auger electron spectroscopy also provides very highspatial resolution (about 10 nm), which makes it especially suitable forsmall feature analysis and elemental mapping. The Pb distribution ofsample Pb02 (the sensor in 150 ppb for two weeks) is mapped using Pb NOOand MNV Auger transition peaks, is displayed in FIGS. 11A-11C. Theleft-up side is the anode while the right-down side is the secondelectrode. As indicated by the higher concentration of lead between theelectrodes, the lead deposited between and connected the electrodes.

Long-Term Monitoring of the Four-Electrode Sensor

Heavy metals can leak into water without of the awareness of users andthus one of the most important features of heavy metal sensors iscontinuous long-term monitoring. To achieve this goal, the sensor needsto be stored or operated in solution for long periods of time and stillfunction normally. The sensors discussed here are ideal for suchoperation because the inert electrodes provide no lifetime limitation.As shown in FIGS. 12A and 12B, both sides of the sensor perform well inSimultap, Tap 1, and Tap 2 after the sensor is operated continuously forfour weeks. The sensor also functions normally after storage in solutionfor two weeks, as shown in FIGS. 13A and 13B. In Tap 1, 15 ppb Pb, and150 ppb Pb solution, the impedances of the sensor on both sides remainssubstantially constant during storage (immersed in the solution withoutany supplied voltage) and the sensor functions normally afteractivation. ΔV₁ on the lead detection side increases significantly(greater than 1V) in both 15 ppb and 150 ppb lead solutions but both ΔV₁and ΔV₂ remain less than 1V in tap water. In FIG. 13C, the sensor isoperated (on) in Tap 1 for two weeks and then stored (off) in Tap 1 foranother two weeks. Hardness precipitates on the cathode during operationmostly dissolved after the storage, thus both the lead detector and theother heavy metal sensors remain unblocked and the sensor can be usedagain.

These experiments suggest that long-term monitoring is possible usingtwo methods. The first approach, as shown in FIG. 13D, is to putmultiple electrode combinations on a single sensor. The surface area ofthe sensor is less than 1 mm², but duplicate sensors can easily beconstructed on this or slightly larger formats. If necessary, wax orother materials can be applied to the sensors during storage to protectthe sensor's surface. The materials can be easily removed just beforeoperation with embedded Ti/Pt heaters to melt and remove the material.The other approach, as shown in FIG. 13E, is to alternate two sensors.One sensor operates for two weeks while the other is immersed in thesame solution with no applied power. The alternation of the sensors canbe programmed and operated automatically, and the sensors withoutapplied power will regenerate through dissolution of precipitated ions.

The response day for 15 ppb Pb solution in FIG. 7 is 7 days but 9 daysafter turned on in FIG. 13C. Therefore, for water safety monitoring,real-time detection of the action level of lead or other dangerous heavymedals is extremely important, and quantification of that level canoccur, for example, off-line.

During long-term monitoring, water temperature influence on the sensoris negligible. For example, lead is the only element that can possiblebe oxidized to a conductive species no matter what the water temperatureis. Also, the dominant redox reactions will not be changed by watertemperature in a residential range (10-50° C.). Water temperature doesnot alter the qualification ability of the sensor. Notwithstanding, thehardness solubility decreases with increasing water temperature. Forexample, the solubility of calcium carbonate changes from 0.53 mM at 25°C. to 0.35 mM at 50° C. Therefore, more hardness may precipitate ontothe cathode side at 50° C., but the lead-detecting anode is notinfluenced due to the pH difference.

The current technology provides sensors that are suitable for long-termmonitoring of lead in drinking water. It is noted that response time canbe shortened by it changing the sensor geometry. The sensing method ofthe sensor is to grow reduced metal or lead dioxide bridges betweenelectrodes in order to change the impedance. Thus by increasing thebridge-formation possibility, the response time is decreased. Somedesigns cause the majority of heavy metal ions to deposit on theelectrodes instead of between the gap. To increase the bridge-formationby metal ions and shorten the response time, the length-to-surface-arearatio and be increased or the gap distance between the electrodes can bedecreased.

The sensor provided by various aspects of the present disclosure candetect contamination of lead or other heavy metals in a variety ofapplications. The four-electrode sensor detects lead on the left twoelectrodes and detect other heavy metals on the right two electrodes.The inert platinum electrode and the experimental results indicate thesensor has a long lifetime, and the sensor can be easily inserted inpipes for continuous monitoring and detection. The sensor can performexcellent qualification, which is important in the monitoring of leadcontamination. Toxic lead exposure causing permanent injuries throughcontaminated tap water has been a concern in the US, and the currentsensor is a solution for detecting such lead outbreaks.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the disclosure. Individual elements or featuresof a particular embodiment are generally not limited to that particularembodiment, but, where applicable, are interchangeable and can be usedin a selected embodiment, even if not specifically shown or described.The same may also be varied in many ways. Such variations are not to beregarded as a departure from the disclosure, and all such modificationsare intended to be included within the scope of the disclosure.

What is claimed is:
 1. A sensor for detecting a heavy metal in water,the sensor comprising: a first electrode and a second electrode, thefirst electrode and the second electrode having complementaryinterdigitated surfaces that are separated from each other by a gaphaving a distance of greater than or equal to about 500 nm to less thanor equal to about 10 μm; first and second leads connected to the firstelectrode and the second electrode, respectively, the first and secondleads being configured to connect the first and second electrodes to apower source; and a resistance connected to the first lead such that avoltage difference across the resistance is indicative of an impedancebetween the first and second electrodes; wherein the sensor isconfigured to continuously monitor the water for the heavy metal bydetecting an increase in the voltage difference arising from a change inthe impedance due to electroplating of a conductive species of the heavymetal on the first electrode or the second electrode.
 2. The sensoraccording to claim 1, wherein the first electrode has a surface area ofgreater than or equal to about 0.4 mm² to less than or equal to about0.5 mm² and the second electrode has a surface area of greater than orequal to about 0.3 mm² to less than or equal to about 0.4 mm².
 3. Thesensor according to claim 1, further comprising: a third electrode and afourth electrode, the third electrode and the fourth electrode havingcomplementary interdigitated surfaces that are separated from each otherby a distance of greater than or equal to about 500 nm to less than orequal to about 10 μm, wherein the second electrode and the thirdelectrode are separated from each other by a gap having a distance ofgreater than or equal to about 1 μm to less than or equal to about 1 mm.4. The sensor according to claim 3, wherein the first electrode is apositive electrode and the fourth electrode is a negative electrode. 5.The sensor according to claim 3, further comprising: a third leadelectrically connected to the third electrode; and a fourth leadelectrically connected to the fourth electrode; wherein the sensor isconfigured such that the first, second, third and fourth leads can beindividually coupled to and decoupled from the power source.
 6. Thesensor according to claim 1, further comprising one or more batteries asthe power source.
 7. The sensor according to claim 1, wherein the heavymetal is lead.
 8. The sensor according to claim 1, wherein the sensor isfree of a reference electrode and a ligand.
 9. The sensor according toclaim 1, further comprising: a substrate to which the first and secondelectrodes are coupled.
 10. The sensor according to claim 9, wherein thesubstrate comprises glass.
 11. A water pipe having an internal boresection through which water flows, wherein the electrode according toclaim 1 is disposed within the internal bore section.
 12. The sensoraccording to claim 1, wherein the electroplating of the first electrodeor the second electrode connects the gap between the first and secondelectrodes such that the impedance decreases.
 13. The sensor accordingto claim 1, wherein the power source provides an electrical potentialsufficient to oxidize the heavy metal in the water into the conductivespecies on the first or second electrode.
 14. The sensor according toclaim 13, wherein the heavy metal is lead and the conductive species islead dioxide.
 15. The sensor according to claim 1, wherein one of thefirst and second electrodes is connected to the power source as an anodesuch that lead in the water is oxidized to form lead dioxide on theanode.
 16. The sensor according to claim 1, further comprising a printedcircuit board to which the first and second electrodes are coupled. 17.The sensor according to claim 1, wherein each of the first and secondelectrodes comprises platinum.
 18. The sensor according to claim 1,wherein the resistance comprises a resistor.
 19. The system according toclaim 18, wherein, in the lead (Pb) sensing configuration, the system isconfigured to detect an increase in the voltage difference arising froma change in the impedance between the first pair of electrodes due toelectroplating of lead dioxide on the electrode of the first pair ofelectrodes connected to the power source during the electroplating mode.20. The system according to claim 18, wherein, in the heavy metalsensing configuration, the system is configured to detect an increase inthe voltage difference arising from a change in the impedance betweenthe second pair of electrodes due to electroplating of a heavy metalconductive species on the electrode of the second pair of electrodesconnected to the power source during the electroplating mode.
 21. Thesystem according to claim 18, wherein the heavy metal sensingconfiguration is configured to detect copper reduced on the electrode ofthe second pair of electrodes connected to the power source during theelectroplating mode.
 22. A system for detecting heavy metals in water,the system comprising: a power source; a resistance connected to thepower source; a lead (Pb) sensor comprising a first pair of electrodesand a first pair of leads to connect respective electrodes of the firstpair of electrodes to the power source via the resistance, the firstpair of electrodes having complementary interdigitated surfaces that areseparated from each other by a gap in a range from about 500 nm to about10 μm; a heavy metal sensor comprising a second pair of electrodes and asecond pair of leads to connect respective electrodes of the second pairof electrodes to the power source via the resistance, the second pair ofelectrodes having complementary interdigitated surfaces that areseparated from each other by a gap in a range from about 500 nm to about10 μm; wherein the system is operable in an electroplating mode in whichone of the first pair of electrodes and one of the second pair ofelectrodes are connected to the power source; wherein the system isfurther operable in a lead (Pb) sensing configuration when the firstpair of electrodes are connected to the power source such that a voltagedifference across the resistance is indicative of an impedance betweenthe first pair of electrodes; and wherein the system is furtherconfigurable in a heavy metal sensing configuration when the second pairof electrodes are connected to the power source such that a voltagedifference across the resistance is indicative of an impedance betweenthe second pair of electrodes.